At present, reflector-type directive antenna systems that typically exploit horn antennas as feeding/receiving systems are used in satellite communications.
Horn antennas fall within the class of aperture antennas that, as is known, are antennas designed to radiate/receive radio signals through radiating/receiving apertures.
In particular, a horn antenna typically comprises:                a hollow metal radiating/receiving element with a rectangular/square/circular cross-section, which                    is known as a horn,            terminates, at a first end, with a radiating/receiving aperture, and            is configured to radiate/receive radio signals through the radiating/receiving aperture; and                        a waveguide, which is coupled to a second end of the radiating/receiving element and which is configured to receive radio signals received by the radiating/receiving element and/or to transmit radio signals to be radiated by the radiating/receiving element.        
An example of aperture antennas is truncated waveguides used in antenna systems to radiate/receive radio signals, for example, in AESA (Active Electronically Scanned Array) antenna systems. In the case of a truncated waveguide, the radiating/receiving element is the end portion of the waveguide where the truncation is made that defines the radiating/receiving aperture.
As is known, satellite communications are implemented on radio channels characterized by bands of radio frequencies that are typically narrower than the operating bands of the horn antennas employed. These antennas are typically designed for wide-band operation, as the operating band of a horn antenna is directly connected to the monomodal bandwidth of the waveguide coupled to the horn.
Thus, a horn antenna, as it is characterized by an operating band typically wider than the radio frequency bands of the satellite channels, received both the narrow-band radio signals transmitted over the satellite channels and the noise present throughout the respective operating band. For this reason, horn antennas are characterized by a high noise figure. Regarding this, a longitudinal section of a traditional horn antenna 10 is shown schematically, and purely by way of example, in FIG. 1 (where the sizes shown are not to scale for simplicity of illustration).
In particular, in the example shown in FIG. 1, the horn antenna 10 is used in reception in a downlink satellite communication, i.e. a satellite communication in which the horn antenna 10 is used by a ground station located on the surface of the Earth (not shown in FIG. 1 for simplicity of illustration) to receive radio signals transmitted by an antenna system installed on board a satellite (not shown in FIG. 1 for simplicity of illustration).
In detail, as shown in FIG. 1, the horn antenna 10 comprises a horn 11 that, in use, picks up, or receives:                a radio signal that has been transmitted by the antenna system installed on board the satellite (henceforth called useful signal, for simplicity of description) and which typically has a narrow-band spectrum S(f); and        the noise that is present throughout the operating band of the horn 11, due to various factors and typically has a wide-band spectrum N(f).        
In addition, always as shown in FIG. 1, the horn antenna 10 also comprises a waveguide 12 that is coupled to the horn 11 and that, in use, receives both the useful signal and noise from the horn 11.
FIG. 2 shows:                the narrow-band spectrum S(f) of the useful signal that is received by the horn 11 and propagates in the waveguide 12; and        the wide-band spectrum N(f) of the noise that is present in the operating band B1 of the horn 11, is received by the horn 11 and also propagates in the waveguide 12.        
Thus, the use of horn antennas in satellite communications entails an undesired increase in antenna noise temperature with a consequent deterioration of the signal-to-noise ratio.
Therefore, in consideration of the large distance between the satellites and the ground stations, atmospheric effects, ground noise and the high noise figure of horn antennas, current satellite communication systems are obliged to use, especially for downlink connections, additional filtering devices and specific signal processing systems designed to maximise the signal-to-noise ratio.